To reduce power consumption of a DC-DC converter, a method is known which switches a semiconductor switch of the DC-DC converter, although the semiconductor switch may cause power-loss of the DC-DC converter. The power-loss of the semiconductor switch is caused by switching-loss (PSW), conduction-loss (PCON), and driving-loss (PDRI). PSW is substantially directly proportional to the size of the semiconductor switch and the amount of the input current. PCON is inversely proportional to the size of the semiconductor switch. PDRI is substantially directly proportional to the size of the semiconductor switch. Thereby, the power consumption of the semiconductor switch depends on the input current and the size of the semiconductor switch. Therefore, a background DC-DC converter includes a plurality of semiconductor switches whose sizes are different from each other, and the converter switches to the semiconductor switch which consumes the lowest power in accordance with the amount of the current into the switches. Thus, the background DC-DC converter can suppress power consumption.
In the above mentioned method, however, it is necessary to calculate the amount of the current input to the switches. In general, a resistor is connected in series between the DC-DC converter and a load, the voltage across the resistor is measured, and the amount of the current can be acquired. In this method, however, some power consumption at the resistor results because of taking the measurement of the voltage across the resistor.